Article: Redescription of Opsipedon gracilis Heer (Crustacea, Tanaidacea) from the Middle Jurassic of northern Switzerland, and the palaeoenvironmental significance of tanaidaceans

Opsipedon gracilis Heer, 1865, is the oldest valid name for a fossil tanaidacean crustacean. The type material, which was originally described as coming from the Lower Jurassic 'Insect marls' (Insektenmergel, Hettangian), is actually of Middle Jurassic age (Opalinum Clay, Aalenian). A redescription of this species is given, and its position within the suborder Apseudomorpha is established. A new model is proposed which relates the occurrence of fossil tanaids with palaeo-oxygenation, both long-term average values and intensity of (seasonal) fluctuations. Many tanaidaceans occur in oxygen-depleted benthic environments, but they cannot tolerate anoxia for more than a few days. Therefore, tanaidaceans are completely wiped out in sedimentary basins that experienced anoxia. After such an event, recolonization proceeds very slowly because tanaidaceans do not possess a planktonic larval stage, and their rate of dispersal as juveniles and adults is exceedingly low. Therefore, fossil tanaidaceans have great potential for aiding distinction between more or less stable dysoxic environments, and environments with recurrent bottom-water anoxia (poikilaerobic biofacies). In the latter case, no fossil tanaidaceans occur.